Self-heating of coal, which can cause self-ignition, is a long-standing problem in the mining, processing, handling, storage, and transportation of this fossil fuel. In spite of many decades of extensive study and research in various parts of the world, no fully satisfactory solution to this problem has been devised to date. The main chemical process involved in the self-heating of coal is the exothermic interaction of atmospheric oxygen with the carbonaceous matter, even though other heat generating processes (such as adsorption or condensation of water onto the coal) can sometimes serve as contributing factors. This atmospheric oxidation of coal will or can start at room temperature. In most cases, this oxidation process represents no hazard because the heat generated is readily dissipated. Only in instances where prevailing conditions favor the accumulation of heat will the coal gradually heat up to higher temperatures (thereby accelerating the rate of oxidation) and eventually ignite spontaneously.
The susceptibility to self-heating is especially pronounced in low rank coals, represented by sub-bituminous coal and lignites. In recent years, certain low rank coals have become commercially important because of their low sulfur content which is usually under 0.5%. Low sulfur coal are an attractive choice for electric power generation in such regions where acid rain problems prevail. In the State of Wisconsin, for example, legislation has been passed that requires sulfur dioxide emissions to be cut in half by the year of 1993. Western low rank coal, particularly from Wyoming, looks promising for reaching this goal. However, this Wyoming coal as mined contains up to 30% moisture and for economic reasons requires drying before shipment. Drying can raise its Btu value, for example, from about 8,000 per pound to 12,500 per pound.
The drying process, usually to less than 10% water, leaves the coal in a virtually pyrophoric state. The coal as it comes out of the dryer readily interacts with atmospheric oxygen up to the point of self-ignition. The commercial use of low sulfur Western coal therefore depends to a large degree on the availability of a dependable and economical method to inhibit self-ignition. Although numerous techniques have been developed and proposed, none of them appears to be satisfactory. Thus, at this time, low rank Western coal is not offered commercially in dried form.
One approach to controlling the self-ignition of low rank coals has been to subject the outer surfaces of the coal particles to a pre-oxidation, using air or oxygen. For example, U.S. Pat. No. 3,723,079 describes a sequential process in which the coal is first over-dried, then treated with oxygen at a temperature of 175.degree. to 225.degree. C., and thereafter partially rehydrated. U.S. Pat. Nos. 4,396,395 and 4,402,706 of 1983 describe apparatus and methods for oxidizing dried low rank coal. In addition to air oxidation, these patents disclose the application of crude oil to further control self-ignition.
The use of pre-oxidation of coal with air is not known to have become a commercial process for treating low rank coal. The addition of crude oil may have been used to some extent in processing of low rank coal. However, prior to the present invention the problem of controlling the self-ignition of low rank coal, such as Wyoming coal, has not received an adequate solution. Low rank coal has therefore continued to be shipped and stored at a much higher moisture content than would be desirable for the most efficient handling, shipping, and burning.
It has been suggested that potassium permanganate (KMnO.sub.4) might be used to pre-oxidize low rank coal. However, no tests are known to have been reported with respect to coal treatment with permanganate for this purpose. It is recognized that when permanganate acts on organic matter, manganese dioxide (MnO.sub.2) is formed. Consequently, it would be assumed that treatment of low rank coal with KMnO.sub.4 would leave residual manganese dioxide on the coal surfaces, and MnO.sub.2 is recognized as a catalyst for oxidation reactions. This consideration may have discouraged experiments with permanganate for pre-oxidation.